Heretofore, in many applications it is desired that a thin, uniform layer or coating of platinum metal be deposited on various substrates because of platinum's well-known properties of having very good electrical conductivity, good corrosion resistance, a high melting point, and being both highly ductile and extremely malleable. Reasons for depositing platinum on a substrate include giving it special catalytic properties, or enhancing its electrical conductivity. Normally, platinum is deposited onto substrates by the vapor deposition technique.
In the prior art vapor deposition technique, a platinum sample in a vacuum oven is bombarded with electrons or other atomic particles, with the substrate being placed some distance away from the platinum source. As the platinum source is bombarded and platinum atoms are ejected from the sample, platinum atoms travel across the chamber, strike the substrate and are deposited on the substrate as metallic platinum.
The vapor deposition technique suffers from a number of drawbacks. That method requires an expensive high vacuum chamber. Additionally, much platinum is wasted when the platinum atoms are ejected from the source because the atoms spread out uniformly in all directions, including the chamber walls, and deposit on both the chamber walls and substrate. For these reasons, vapor deposition suffers from the limitations of high operational costs, the need for specialized equipment, generally poor adherence of deposited platinum to the substrate and being non-directional.
There is also a prior art aqueous method for deposition of platinum in which the substrate is placed in a solution containing platinum ions. Platinum ions in solution could, by reacting with a specific oxidizing agent, or electrons, be reduced to platinum metal directly on the desired substrate. This method would not waste as much platinum and would be less expensive. One use of the aqueous deposition method is passing electrical current through a chloroplatinic acid solution, to provide negatively charged electrons, which convert platinum ions in the +4 state into neutral, uncharged, metallic platinum. However, that method needs an electrically conductive substrate, and if the substrate is a nonconductive material such as glass, this method would not work.
The problems, drawbacks and limitations of the vapor deposition technique and the aqueous deposition procedure for conductive substrates can be overcome by a new procedure for depositing platinum. It is well-known that when formaldehyde is added to a chloroplatinic acid solution, platinum metal, in the form of small particles, can precipitate out of the solution. If a glass slide is placed in such a solution while formaldehyde is added, very little platinum sticks to the glass slide. If the solution of chloroplatinic acid also contains a suspension of carbon black powder, and then formaldehyde is added, platinum deposits primarily in, or on, the carbon black powder suspended in the solution. If a glass slide is present in this solution containing carbon black powder suspended in chloroplatinic acid, and then formaldehyde is added, the platinum deposits on the carbon black and not on the glass. These results and related findings are set forth in TABLE I, further below.
The present invention solves the non-directional and other drawbacks of the vapor and aqueous deposition techniques by providing a method for coating nonconductive substrates with platinum having the steps of immersing the substrate, such as a glass slide, in chloroplatinic acid also containing a suspension of carbon black powder treated to make it hydrophobic, thereby allowing the hydrophobic carbon powder to catalyze and drive the reduction of platinum towards the substrate so that the chloroplatinic acid then forms metallic platinum on the glass substrate, rather than on the carbon black powder. The method of the present invention is an effective, inexpensive and simplified procedure for platinizing nonconductive substrates that does not suffer from any of the disadvantages and limitations of other platinum deposition techniques. Although the inventors have discovered the advantages of this technique by immersing a glass slide in the requisite solution, these results should also be achieved for other nonconductive substrates, therefore any nonconductive substrate is considered within the scope of this disclosure and the appended claims.
The following definition should be considered applicable to this invention:
"hydrophobically treated" is defined as treatment of carbon black powder with either CF.sub.4 gas plasma or any other gas plasma or chemical treatment that renders the carbon black powder more water repellent.